The present invention relates to a gas barrier film, in more detail, a gas barrier film having high impermeability to gas (hereinafter referred to as "gas barrierability") at high humidity and excellent in its adhesiveness to a thermoplastic resin substrate and post-lamination gas barrierability.
In the field of packaging for foods and chemicals, etc., ongoing developments have been made with a view to providing films with gas barrierability capable of preventing the ingress of open air, since any ingress of oxygen, etc. from open air does not allow long-term storage of contents due to deterioration.
According to Polymer Engineering and Science, (December 1986), vol. 20, No. 22, P. 1543.about.1546, conventionally developed films with gas barrierability include those of polyvinylidene chloride, polyacrylonitrile and polyvinyl alcohol. However, since polyvinylidene chloride contains chlorine atoms and polyacrylonitrile contains CN groups, then especially more recently, waste materials resulting from their preparation and use have caused serious environmental problems. Furthermore, in particular, since polyvinyl alcohol contains --OH groups, the gas barrierability of polyvinyl alcohol greatly depends on humidity and becomes remarkably low at high humidity. Similarly, the ethylene-vinyl alcohol copolymer, developed for reducing the humidity dependence of polyvinyl alcohol, still has insufficient gas barrierability at high humidity.
On the other hand, films having a vapor-deposited inorganic material such as silicon oxide (JP-B-53-012953, etc.) or aluminum oxide (JP-A-62-179935, etc.) on the surface of a substrate were developed. However, these films have disadvantages in that the vacuum evaporation necessary for film formation raises the cost significantly and the film is hard to handle because of the inflexible inorganic coating and its poor adhesiveness to the substrate.
As a means for solving these problems, a film having a coating consisting of a metal oxide and polyvinyl alcohol formed on a substrate (JP-A-56-004563, etc.) was developed, but its gas barrierability at high humidity is still not satisfactory. Furthermore, films having a gas barrier layer consisting of inorganic laminar compounds and a highly hydrogen bondable compound (JP-A-06-093133, JP-A-07-041685, etc.) have been proposed, but the films are very disadvantageous in their productivity because long time drying or heat treatment is necessary in the step of forming a gas barrier layer with high gas barrierability, elongation gas barrierability, durability of gas barrierability and moreover, the adhesiveness that can be achieved between the coating material and the substrate is low. The elongation gas barrierability is a gas barrierability under elongation. If the elongation gas barrierability is high, gas barrierability would not deteriorate even if the film is stretched at the steps such as printing, dry laminate or package formation. The durability of gas barrierability is a gas barrierability after the film is subjected to a repeated twist and if this durability of gas barrierability is high, gas barrierability would not deteriorate even if the film is subjected to a vending at package formation or even if wrinkles are formed after package formation. If a film could be produced which had a gas barrier layer which adhered strongly to a substrate then a package, such as a bag, produced from such a film would be highly reliable because breakage or deformation would not occur and because the gas barrierability after lamination would not significantly deteriorate.
The present invention addresses the above mentioned problems of conventional gas barrier film and seeks to provide a solution by providing a gas barrier film comprising a substrate having thereon a gas barrier coating, which film has gas barrierability even at high humidity and under elongation and also has durability of gas barrierability and with adhesiveness of the gas barrier coating to the substrate and post-lamination gas barrierability.